TIJUANA  The trial of a U.S. Marine veteran detained after crossing into Mexico with weapons and ammunition focused Monday on the testimony of two customs inspectors who performed the initial search of his vehicle.

Andrew Tahmooressi, 25, has maintained that he never intended to leave the United States on March 31 when he ended up at the El Chaparral border crossing in Tijuana, unable to make a U-turn. Inspectors found more than 400 rounds of ammunition and three loaded weapons — all of which Tahmooressi has said he purchased legally in the United States.

Tahmooressi’s attorney, Fernando Benítez, worked to show Monday that his client’s constitutional and civil rights were violated when customs officials kept him for close to eight hours without the assistance of an attorney, a representative of the U.S. Consulate or a proper translator. Benítez said his client should have been promptly turned over to federal prosecutors.

“We are beyond any reasonable doubt now convinced Mr. Tahmooressi’s human rights were trampled under the pretense of following administrative protocol for a customs procedure,” Benítez said as he exited the federal courthouse in Tijuana’s Río Zone. “They may have followed customs protocol ... (but) they violated the constitution, and they violated federal criminal code.”

Police sirens blared as Tahmooressi arrived at the courthouse shortly before 9 a.m. from El Hongo State Penitentiary outside Tecate. His mother, Jill Tahmooressi, slipped in quietly through a side entrance, as did representatives of the U.S. Consulate.

The case of the Florida native has generated intense interest among veterans groups and conservative talk show hosts demanding that the Obama administration press Tahmooressi’s cause with Mexican authorities.

A petition to the White House on Tahmooressi’s behalf has accumulated more than 133,000 signatures. U.S. Rep. Duncan Hunter, R-San Diego, and other members of Congress have written letters on his behalf, pleading for leniency for Tahmooressi, who served two tours of duty in Afghanistan and was beginning treatment for post-traumatic stress disorder in San Diego.

But the Mexican government has insisted that Tahmooresi’s case is neither a diplomatic or political issue, saying in a statement that it is “strictly a judicial issue that will be resolved by the Mexican federal courts.”

Media were barred from attending Monday’s hearing before Judge Víctor Octavio Luna Escobedo of the Sixth District Court. The session lasted for close to five hours and included testimony from two soldiers who were present at El Chaparral at the time of Tahmooressi’s detention.

The two customs officers, who have not been named, hired a private Tijuana attorney, Javier López, to represent them in the proceeding. López said the duration of the detention by customs officials violated no procedural rules. “It could be 12 hours; it’s not an issue,” López said.

Mexican law “is very clear,” López added, saying that “when anybody crosses that border with guns, it is illegal.”

Benítez said he is striving to show that his client had no criminal intent when he crossed. “We have to go for broke in this case. We need to prove his innocence,” Benítez said. But Benítez is simultaneously working to have the court dismiss the seized weapons and ammunition as evidence on the grounds that they were illegally seized.

Benítez is the third attorney hired by Tahmooressi’s family. He has said that previous counsel missed key opportunities to seek dismissal of the case during the first days after Tahmooressi’s arrest.

Estimates of Tahmooressi’s possible sentence, if found guilty, have ranged from seven to 21 years. Benítez said the trial “is going to take a few more months.”